The goals in rocket motor design are a large propulsive impulse and a large impulse-to-weight ratio, while extending the impulse over time to produce the maximum range for a given payload. The propellant must burn in a predetermined geometric pattern at a controlled rate and continue to do so until the propellant is fully consumed, to avoid excessive increases in temperature which might lead to premature explosions or destruction of the motor case. Propellant grains which achieve this most effectively are those which have a rubbery consistency which resists the formation of discontinuities such as cracks in the body of the grain and gaps between the grain and the case wall. Such discontinuities create an explosion danger by increasing the surface area where solid propellant is exposed to the advancing burning front. Burn rate and burn efficiency are also affected by the proportions of the oxidizer, binder and fuel in the propellant grain and by the uniformity of the grain composition. On the whole, therefore, the composition, consistency and shape of the propellant grain, favorable mechanical properties and the means by which the grain is bonded to the motor case are important factors in achieving the safety, reliability and favorable mechanical properties which are desirable in rocket motors.
Conventional methods of fabricating composite case rocket motors begin with the fabrication of the motor case over a mandrel in the shape of the propellant grain, the mandrel being either a disposable structure made of plaster or sand, or an assembled structure which can be dismantled and removed in small size parts. Before the motor case is applied, the mandrel is coated with an insulating material, either by tape-winding or by the use of a premolded form of insulation. Once this is done, the motor case itself is formed by winding the insulation-coated mandrel with filament which is either graphite or polyethylene fiber pre-impregnated with an epoxy resin. The wound filament is then cured to form a solid shell. The mandrel is then removed from the cured shell and replaced with liquid propellant material which is cast and cured in place to form the propellant grain, using a core rod of appropriate configuration to form the center perforation of the grain. Unfortunately, removal of the mandrel is a labor-intensive and costly procedure, and when combined with the numerous other steps and stages in the overall procedure, results in many opportunities for error, damage and loss.